The present invention relates to methods and compositions for reducing the amount of water produced from a subterranean formation, and, more specifically, to methods and compositions for treating at least a portion of a subterranean formation to reduce water permeability using a gellable treatment fluid that comprises a gel-time modifier comprising a quaternary ammonium salt.
Water often undesirably accompanies the production of oil and gas from a well penetrating a subterranean formation. The unwanted production of water from hydrocarbon-producing wells can constitute a considerable technical problem and expense in oilfield operations. If the ratio of produced-water to produced-oil and gas becomes sufficiently large, the cost of separating the water and disposing of it can become a barrier to continued production. This can lead to abandonment of a well penetrating a subterranean formation, even when significant amounts of hydrocarbons remain therein.
In a subterranean formation, water's high mobility often allows it to flow to or from a well bore by way of natural and manmade fractures, high permeability zones, and the like. In such cases, less permeable zones in the formation can be bypassed. The bypassing of less permeable zones can be especially problematic when an aqueous treatment fluid is introduced into a subterranean formation. For example, in enhanced oil recovery techniques, an aqueous fluid can be introduced into a subterranean formation during water flooding operations. When less permeable zones are present in the subterranean formation, lower oil and gas production can occur due to a less effective water flooding operation being realized. The presence of natural and manmade fractures, high permeability zones and the like also poses problems when aqueous fluids need to be introduced into low permeability zones for purposes other than flooding operations. Examples can include acid stimulation treatments and near-wellbore cleanup fluids. In such cases, aqueous fluids can preferentially enter high permeability zones and bypass low permeability zones, which are the intended targets of fluid treatments.
One way in which the foregoing problems can be addressed is through conformance control treatments, whereby high permeability zones become fully or partially blocked to fluid flow, in the case of unwanted water production, full blockage of water producing permeable zones, regardless of high or low permeability, can stop the unwanted production of water. In the case of flooding operations, partial blocking of high permeability zones can enable oil production from bypassed low permeability zones. In the case of stimulation and near wellbore cleanup, partial blocking of high permeability zones can allow diversion of a stimulation fluid (e.g., an acid) or well cleanup fluid to a low permeability zone.
Conformance control treatments can involve introducing gettable polymer systems into a subterranean formation via an aqueous treatment fluid. The gellable polymer systems can form a gel through crosslinking a water-soluble polymer using a crosslinking agent. The gel-time and the gel strength of the gettable polymer systems are among the factors that can determine the effectiveness of a conformance control treatment. For example, if the gel-time is too short, introduction or placement of the gettable polymer system into a subterranean formation can prove problematic. Conversely, if the gel-time is too long, the gellable polymer system may not form a gel in the desired portion of the subterranean formation, or long waiting periods may be required before further operations can be carried out.
A number of crosslinking agents can be used to crosslink water-soluble polymers in gellable polymer systems. Chromium and other transition metal ions can be used to crosslink acrylamide-containing polymers and copolymers. Generally, gels formed using such crosslinking agents have proven unsuitable at higher temperatures (e.g., above about 80° C.) due to uncontrolled crosslinking rates (e.g., short gel-times), crosslinking agent precipitation, polymer degradation, and the like. In addition, chromium and certain other transition metal ions can have an undesirable environmental impact. Acrylamide-containing polymers, copolymers, and partially hydrolyzed variants thereof can also be gelled with polyalkyleneimines and polyalkylenepolyamines. In such gellable polymer systems, the gel-times are often so short that the crosslinking agent and water-soluble polymer are generally pumped downhole separately in order to prevent premature gellation from occurring. Gel-time accelerators and gel-time retarders have also been used in the art to modify the gel-times in such systems.
Gellable polymer systems typically comprise a crosslinkable polymer and a crosslinking agent. Normally, as the concentration of either of these components decreases in a treatment fluid, the time required to form a gel as measured by an increase in viscosity of the treatment fluid at a given temperature, referred to herein as “gel-time,” increases. Typically, the gel-time is determined by measuring the viscosity of a treatment fluid comprising the gettable polymer system as a function of time. Although treatment fluids having lower concentration gettable polymer systems are desirable from a cost-of-goods standpoint, increased gel-times at lower concentrations can make such treatment fluids ineffective for treating a subterranean formation.
The gel-time of a treatment fluid comprising a gellable polymer system is usually a function of temperature and the concentrations of water-soluble polymer and crosslinking agent therein. Generally, at higher concentrations of these components, shorter gel-times can result. Conversely, at lower concentrations, gel-times can be increased. In some instances, low concentration gellable polymer systems can have gel-times that are increased to such an extent that they can become ineffective for treating a subterranean formation. Furthermore, at lower concentrations, the gel strength can also be impacted to some degree. Although gel strength is not typically a concern in most conformance control treatment fluids due to relatively high concentrations of water-soluble polymer and crosslinking agent being used, it bears mentioning that gel strength can be reduced in low concentration gellable polymer systems.
In conformance control treatments using acrylamide-containing polymers and copolymers and crosslinking agents such as, for example, polyethyleneimine and polyalkylenepolyamines, relatively high concentrations of both components are typically used. From an economic standpoint alone, it would be desirable to reduce the amounts of either of these materials while still maintaining acceptable gel-times and gel strengths to achieve successful conformance control. For polyethyleneimine, in particular, it would also be desirable to reduce the amounts of this highly corrosive material being used in conformance control treatment fluids in order to improve their environmental rating.